This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-299131, filed Sep. 28, 2001, the entire contents of which are incorporated by reference.
1. Field of the Invention
This invention relates to a carbon dioxide gas absorbent and a carbon dioxide gas separating apparatus.
2. Description of the Related Art
If it is desired to perform the separation and recovery of carbon dioxide gas from the exhaust gas that has been discharged from a combustion apparatus such as an engine where a fuel containing mainly of hydrocarbons is combusted, it is effective to perform the separation and recovery of carbon dioxide gas at a location which is close to the combustion chamber and therefore is high in the concentration of carbon dioxide gas. Since this location which is close to the combustion chamber is relatively high in temperature, unless the exhaust gas discharged from the combustion chamber is cooled by a heat exchanger, etc., the separation and recovery of carbon dioxide gas is inevitably required to be performed under an environment of about 300xc2x0 C. or more.
As for the method of separating and recovering carbon dioxide gas, there have been conventionally proposed various methods such as a method where cellulose acetate is employed, a method where an alkanol amine type solvent is employed, and a chemical absorption method where carbon dioxide is absorbed by using a solution. In any of these conventional carbon dioxide gas separation/recovery methods, the absorption of carbon dioxide gas is performed at a temperature of about 200xc2x0 C. or less. Because of this, the temperature of the exhaust gas containing carbon dioxide gas on the occasion of introducing it into an absorption system is required to be controlled to be about 200xc2x0 C. or less. Therefore, the exhaust gas having a high concentration of carbon dioxide gas and existing in the vicinity of the combustion chamber is required to be cooled to a temperature of about 200xc2x0 C. or less by a heat exchanger, etc. before the carbon dioxide gas is to be separated and recovered. As a result, there is a problem that the energy consumption for the separation and recovery of carbon dioxide is inevitably increased.
On the other hand, Jpn. Pat. Appln. KOKAI Publication No. 9-99214 discloses a carbon dioxide gas absorbent comprising lithium zirconate. Further, Jpn. Pat. Appln. KOKAI Publication No. 2000-262,890 and Jpn. Pat. Appln. KOKAI Publication No. 2001-170,480 discloses a carbon dioxide gas absorbent comprising lithium silicate. Lithium zirconate and lithium silicate are capable of absorbing carbon dioxide gas at temperatures exceeding about 500xc2x0 C. Further, when lithium zirconate and lithium silicate are heated to a temperature of 600xc2x0 C. or more, carbon dioxide is desorbed therefrom. Moreover, when at least one kind of alkali carbonate selected from the group consisting of lithium carbonate, potassium carbonate and sodium carbonate is added to lithium zirconate and lithium silicate, the carbon dioxide gas-absorbing property of the carbon dioxide gas absorbent can be improved, so that it becomes possible to effectively absorb carbon dioxide gas even if carbon dioxide is of low concentration.
However, these conventional carbon dioxide gas absorbents incorporating alkali carbonates are accompanied with problems that the serviceable life thereof is relatively short, and that if the separation and recovery of carbon dioxide gas are to be performed using these conventional carbon dioxide gas absorbents, it requires a relatively high temperature for releasing carbon dioxide gas from the carbon dioxide gas absorbent.
Therefore, an object of the present invention is to provide a carbon dioxide gas absorbent which is long in serviceable life.
A further object of the present invention is to provide a carbon dioxide gas separating apparatus which is capable of effectively performing the separation and recovery of carbon dioxide gas.
According to one aspect of the present invention, there is provided a carbon dioxide gas absorbent comprising:
lithium silicate;
0.5 mol % to 4.9 mol % of alkali carbonate per mole of the lithium silicate; and
at least one element selected from the group consisting of aluminum, magnesium, calcium, iron, titanium and carbon.
According to another aspect of the present invention, there is provided a carbon dioxide gas separating apparatus comprising:
a reaction chamber having a carbon dioxide gas inlet and a product gas outlet;
a carbon dioxide gas absorbent placed in the reaction chamber; the carbon dioxide gas absorbent comprising lithium silicate; 0.5 mol % to 4.9 mol % of alkali carbonate per mole of the lithium silicate; and at least one element selected from the group consisting of aluminum, magnesium, calcium, iron, titanium and carbon; and
a heater heating the reaction chamber and disposed around the reaction chamber.